In industrial processes that treat and process slurries containing solid matter, there is often a need to regularly and continuously control the process on the basis of the element contents of the solid matter in the slurry. It is well-known to use certain analysis methods in analyzing slurries that contain solid matter. These include optical methods, nuclear magnetic resonance, and prompt gamma spectroscopy as well as methods utilizing X-rays, such as the method based on X-ray fluorescence. In order to optimally observe and control the industrial processes on the basis of such measurement results, samples are taken continuously from the process flow and analyzed with a delay, which is significantly smaller than the time constant of the process. Mineral separation and hydrometallurgical processes are examples of industrial processes, wherein a real-time analyzing of slurries and liquids are required. Flotation, magnetic and gravity separation, extraction of metals, cleaning of liquid, as well as electrolytic cleaning and recovery processes represent mineral and hydrometallurgy processes that use on-line analyzers.
Laser-induced breakdown spectroscopy (LIBS) is an optical method for performing elemental concentration measurements. LIBS includes generating laser pulses that may be focused toward a sample, such as onto a surface of a sample (e.g., solid or liquid) or into a sample (e.g., liquid or gas). The laser pulse exhibits a high enough power density to transform at least a part of the sample into a state of a plasma. Emitted light from the plasma plume is collected using light collection optics, and the spectral distribution (i.e., intensity as a function of wavelength) of the collected light is analyzed with a spectrometer by generating electronic information describing the spectral distribution of the collected light. Because atomic and molecular constituents of sample materials exhibit a characteristic optical emission spectrum, the information generated by the spectrometer forms a “fingerprint” of the sample material, revealing the constituents of that part of the sample onto which the laser beam was focused. LIBS can provide an easy, fast, and in situ chemical analysis with a reasonable precision, detection limits, and cost.
A prior art arrangement for online analysis of the chemical composition of process flow material with LIBS is disclosed in “On-Stream Analysis (OSA) of Industrial Slurries for Process Control and Optimization Using Laser-Induced Breakdown Spectroscopy (LIBS)”, Louis Barrette et al, Proceedings of 36th Annual Meeting of the Canadian Mineral Processors, Paper 17, January 2004. In the prior art arrangement the laser is focused on a steady flow of free-falling slurry. The industrial slurry flow is sampled in three steps. In the first stage, a commercial sampler extracts a portion of the process flow. At the secondary sampling stage, the slurry is conditioned for both flow and density and fed to the injector in such a way to get a smooth free-falling flow suitable for laser sampling. This step is often referred to as a sample presentation. The laser pulse constitutes the final sampler: through energy absorption by the target material, it extracts a μg sample as a short-lived plasma that is analysed with spectroscopic techniques. The output flow is collected and returned to the process. A modified prior art arrangement is disclosed in “Shooting Slurries with Laser-Induced Breakdown Spectroscopy: Sampling is the Name of the Game”, Daniel Michaud et al, Applied Optics, Vol. 42, Issue 30, pp. 6179-6183 (2003). The modified sampler geometry, which is shown in FIG. 1, consists of a reservoir 2 with a mechanical stirrer 3, a double-head peristaltic pump 4, a laboratory faucet 5 shaped like an upside-down J, and a rigid receiver tube 6. One end of the receiver tube 6 slips tightly over the tip of the faucet 5; the other end returns the slurry to the reservoir 2. The laser 7 aims the 8-mm-diameter free-falling slurry column through a hole in the receiver tube 6 at a point situated 5 mm below the tip of the faucet 6. Near the strike point, downward aspiration (vacuum) 8 is provided to evacuate nebulized material that results from the laser impact: Inclusion of the upside-down-J-shaped faucet 5 is important to ensure good flow quality minimizing exit splashing. The new sampler has proved successful in circulating high-density iron ore slurries which tend to sediment as well as low-density graphite slurries which tend to float.
These prior art arrangements require very accurate positioning of the laser beam and are sensitive to the surface fluctuation variation of the free-falling slurry flow. The LIBS analysis results have been found to exhibit a strong dependence on the slurry solids content and particle size, when using the described arrangements. With coarser particles and with smaller solids content the sensitivity of the analysis is significantly reduced. Special measures like vacuum suction air flow have been found to be necessary to keep the optical components clean from sample nebulized by the laser pulse.